The present invention relates to the production of watertight and thermally insulating tanks built into a bearing structure, especially the hull of a ship, intended for the transportation by sea of liquefied gases and, in particular, the transportation of liquefied natural, gases with a high methane content.
In French Patents 1,438,330, 2,105,710, 2,146,612 and 2,683,786, there has already been described the production of a watertight and thermally insulating tank of this type, the said tank consisting of two watertightness barriers in succession, a primary one in contact with the liquefied gas transported and a secondary one located between the primary barrier and the bearing structure, these two watertightness barriers being alternated with two layers of thermal insulation termed "insulating barriers". In these embodiments, the primary and secondary insulating barriers are made up of parallelepipedal compartments filled with lagging and the primary and secondary watertightness barriers are formed of metal strakes, for example made of invar, with turned-up edges and welded on either side of a weld flange.
It has also been proposed, for example in French Patent 2,462,336 and 2,504,882, to produce at least one of the insulating barriers by means of a layer of cellular material. However, it has been observed that it was desirable for the primary insulating barrier to include, towards the inside of the tank, a rigid plate giving better resistance with regard to the impacts which arise on the walls of the tank through the movements of the liquid during transport, which movements are due to the rolling and pitching of the ship, and this technique was adopted, especially, in French Patent 2,504,882.
However, in all cases, there was a problem relating to the fastening of the primary barriers which have to be held elastically bearing against secondary barriers. In French Patent 2,462,336, the primary barrier was fastened to the secondary barrier without any joining to the bearing structure of the ship, and this was highly advantageous as regards the insulation characteristics, but exhibited drawbacks as regards the cost price and the ability to purge the primary insulating barrier. French Patent 2,504,882 proposed to fasten the primary insulating barrier directly to the bearing structure of the ship using anchoring members which passed through the secondary watertightness barrier however, this technique generated stress concentration zones in the secondary watertightness barrier and set up a direct thermal bridge between the primary barrier and the bearing structure, which was unfavourable from the insulation performance point of view.
Furthermore, it is known that, for a constant tank thickness, it is beneficial to increase the thickness of the secondary insulating barrier to the detriment of the primary insulating barrier because, should there be a leak at the primary watertightness barrier, the accidental cold zone is all the further away from the double hull as the secondary insulating barrier becomes thicker. Unfortunately, the reduction in thickness of the primary insulating barrier runs into a difficulty which is due to the structure of the angles for connecting the walls of the tank together in the zones where the transverse bulkheads of the ship meet the double hull; in actual fact, according to the techniques described, the connecting corner was produced in the form of a ring, the structure of which remains constant right along the solid angle of intersection of the said transverse bulkhead with the double hull of the ship as described, for example, in French Patent 2,683,786. It is known that when the ship is moving in heavy seas, the deformation of the girder which forms it generates very significant tensile stresses at the primary and secondary watertightness barriers, which stresses, in fact, add to the tensile stresses generated in these watertightness barriers when the tank is subjected to cold. The connecting ring must of course allow these forces to be taken up by the bearing structure and, in French Patent Application 93-10720, a structure has been proposed which allows these forces to be taken up while adopting a thin primary insulating barrier.
In the state of the art which has just been described, it is observed that the various problems mentioned hereinabove have still been solved one by one by describing structures in which the primary and secondary watertightness barriers each consist of an assembly of metal strakes made of invar welded to one another with turned-up edges. Each of these two watertightness barriers has to be held bearing elastically on its associated insulating barrier on the one hand, avoiding thermal bridges to the bearing structure and, on the other hand, without giving rise to prohibitive production costs. In all the embodiments described in the state of the art for these tanks with a double watertightness barrier, the primary and secondary insulating barriers are independent and fitted separately.